Electrical optical sensing devices are well known in the field of liquid level sensors. They generally employ reflective and refractive optical theory based on the different indices of refraction between two media.
One such sensor is described in U.S. Pat. 4,711,126, which issued to Houpt et al. on Dec. 8, 1987. Houpt teaches a Y-shaped light probe having a single light source at one arm of the Y, and a single light detector at the other. The wall of the probe is coated with a reflective material, preferably gold, with windows in the form of thin rings cut out in the reflective material. As the light travels from the light source to the bottom of the sensor, and reflects back to the detector, some of the light is lost through the windows to the outside liquid by refraction. The liquid level is determined by the amount of light that reflects and travels back to the detector. Houpt's sensor has several disadvantages. It is costly to make, requiring gold coating of the entire probe, and milling to achieve thin window cutouts in the gold coating. Another disadvantage is that this sensor gives false readings if a thin film of liquid forms on the exterior of the probe, covers some or all of the window slits which causes the light rays to refract out of the probe and into the liquid inaccurately. A similar fault is also encountered if contaminants and impurities in the liquid attach to the probe which can affect the amount of light returning to the detector. Sensors of this type are also not practical for liquid level sensing in relatively deep tanks. On the contrary, the instant invention employs refractive and transmissive properties of optics, and does not require coating of the sensor for reflective purposes. Unlike Houpt's liquid level sensor, which requires recoating and remilling the sensor probe, the present invention is easy to modify to adapt to changing applications.
Another optical liquid level sensor is described in U.S. Pat. 3,995,169, issued to Oddon on Nov. 30, 1976. This invention teaches a sensor consisting of a plurality of U-shaped light pipes of various length, each with a light source and a light detector at respective ends of the pipe. The geometry and construction of the pipe is such that the light reflects back to the detector end if liquid is not present at the U bend of the pipe, and refracts out of the pipe if liquid is present. This liquid level sensor is good for reasonably deep tanks and is less expensive, but it still has some serious drawbacks. The light pipes are not of ruqqed construction, are bulky and cumbersome, and the number of liqht pipes one can have in the sensor is limited. Therefore, sensors of this type is good only for coarse measurements and not for liquid level sensing of fine resolution. This sensor also has the same problem as Houpt's sensor in that the sensor would give false levels if liquid or impurities and contaminants attach to the pipes. The instant invention overcomes the aforementioned disadvantages. The multi-point sensor is of more rugged construction and more compact. Having an array of sensing elements, with their outputs connected in parallel to a single node, the instant invention can monitor liquid level to fine resolution.
In the U.S. Pat. 4,354,180 issued to Harding on Oct. 12, 1982, an electro-optical liquid level sensor is described. Harding teaches a sensor also of probe type configuration, using reflection or refraction of a light beam to determine the liquid level. This sensor detects only at a single level, usually for maximum level or minimum level indication, rather than multi-point liquid level monitoring capability of the present invention.
Each of the above identified electrical optical liquid level sensors and others in the field is adequate in only certain situations, but has serious pitfalls in the applications to which the instant invention is directed. The present invention is directed to overcome the problems as set forth above.